Immediately after bacteriophage T4 infects Escherichia coli host RNA polymerase transcribes a region of the phage genome containing the vs locus. The mRNA is translated and produces a peptide which rapidly forms a strong complex with tRNA1val. The physiological significance of this modification is elusive since a bacteriophage T4 mutant has been isolated which does not modify host valyl-tRNA synthetase but which grows normally on laboratory strains of E. coli. Our goals during the current budget period are threefold. We intend, first, to attempt isolation of an E. coli mutant which is incapable of growing T4 mutants impaired in the modification reaction. Second, we seek to utilize antisera prepared in our laboratory to develop a radioimmune assay for the peptide tau. Finally, we seek to demonstrate the in vitro synthesis of tau in a system primed with T4 DNA and evaluate the effect of the modification reaction on proteins synthesized in vitro. BIBLIOGRAPHIC REFERENCES: Published: McClain, W.H., G.L., Marchin, F.C. Neidhardt, K.V. Chace, M.L. Rementer, and D.H. Hall. 1975. A gene of bacteriophage T4 controlling the modification of valyl-tRNA synthetase, Virol. 67: 385-394. Muller, U.R. and G.L. Marchin. 1976. Properties of a T4 Bacteriophage Peptide that Modifies E. coli Valyl-tRNA Synthetase. Abtr. Ann. Meet. Am. Soc. Micro. (in press).